The present invention relates to semiconductor devices that provide conductivity modulation and serve as semiconductor switching elements. More particularly, the present invention relates to a semiconductor device that provides conductivity modulation and serves as a semiconductor switching element for power conversion or control equipment, as well as a method for controlling such a semiconductor device.
Among conventional devices, semiconductor devices used for power conversion or for control equipment are required to have a small voltage drop when turned on. This feature is necessary in order to minimize power loss. Accordingly, a thyristor or an insulation gate bipolar transistor (hereinafter referred to as IGBT) is suitable for applications that require a high-voltage blocking capability.
Referring to FIG. 8, a sectional view of a basic structure for describing the configuration and operation of the IGBT is shown. This lateral structure allows an IGBT to be incorporated on the output side of an integrated circuit. In an actual semiconductor device, a plurality of such structures are placed in parallel along one plane.
The IGBT of FIG. 8 is formed by forming an n-type high-resistivity n- layer 2 laminated by, for example, an epitaxial method on a p-type silicon substrate 1, forming a p-type (p) base region 9 on part of the surface layer of the n- layer 2 by diffusing impurities from the surface. The next step is forming an n+ source region 10 on part of the surface layer of the p-type (p) base region 9, similarly by diffusing impurities from the surface.
The process then involves providing a second gate electrode 12 connected to a second gate terminal G2 via a second gate oxide film 11 on the surface of the part of the p-type (p) base region 9 between the n- layer 2 and the n+ layer source region 10. Additionally, a source electrode 19 connected to a second main terminal T2 commonly contacts the n+ source region 10 and the (p) base region 9. On the right-hand side of the figure, a p+ drain region 22 is similarly formed by diffusing impurities from the surface, and a drain electrode 23 connected to the first main terminal T1 contacts p+ drain region 22.
This lateral IGBT can be turned on or off by applying a voltage to the second gate electrode 12. When a positive voltage is applied to the second gate electrode 12 from the second gate terminal G2 with a forward voltage applied to the IGBT, (i.e., a voltage that is positive relative to T2 is applied to a main terminal T1), electrons (e) that are majority carriers flow into the n- layer 2 from the n+ source region 10 via an inversion layer formed on the surface of the p-type (p) base region 9 directly below the second gate electrode 12, and then flow into the p+ drain region 22 through the forwardly biased junction between the n- layer 2 and p+ drain region 22.
Therefore, according to this device, the structure including the (p) base region 9 on the source electrode 19 side of the IGBT controls the injection of majority carriers. This current corresponds to the base current of a pnp transistor having the (p) base region 9 as a collector, the n- layer 2 as a base, and the p+ drain region 22 as an emitter. Therefore, when this bipolar transistor is turned on, a collector current flows from emitter to collector. That is, the current flows from p+ drain region 22 to p-type (p) base region 9 to conduct electricity between the main terminals T1 and T2. When electrons (e) are injected into the p+ drain region 22, many holes (h) are reversibly injected from p+ drain region 22 to n- layer 2, thereby providing conductivity modulation. This effect makes the ON-state voltage between the main terminals T1 and T2 substantially lower than that of a normal MOSFET.
The IGBT can be turned off simply by removing the voltage of the second gate electrode 12 to eliminate the inversion layer on the surface layer of the p-type (p) base region 9 directly below the second gate electrode 12, thereby stopping majority carriers (e) from flowing into the n- layer 2. After the IGBT is turned off, the depletion layer widens inside the n- layer 2 to stop charged atoms.
Referring to FIG. 9, a sectional view of the basic structure of a lateral MOS control thyristor (hereinafter referred to as an MCT) that can be turned off by a MOS gate is shown. An n-type high-resistivity n- layer 2 is laminated by, for example, an epitaxial method on a p-type silicon substrate 1, and a p-type (p) base region 9 is formed on part of the surface layer of the n- layer 2 by diffusing impurities from the surface thereof. An n-type (n) base region 14 is formed on part of the surface layer of the p-type (p) base region 9, also by diffusing impurities from the surface. A p+ cathode region 15 with an impurity concentration higher than that of the p-type (p) base region 9 is formed on part of the n-type (n) base region 14.
A cathode electrode 13 connected to the second main terminal T2 is provided to contact both the p+ cathode region 15 and the n-type (n) base region 14. A second gate electrode 12 is provided via a second oxide film 11 on the surfaces of both the n-type (n) base region 14 and p-type (p) base region 9 between the n- layer 2 and the p+ cathode region 15.
On the right-hand side of the figure, a p+ drain region 22 is similarly formed by diffusing impurities from the surface, and a drain electrode 23 connected to the first main terminal T1 contacts the p+ drain region 22.
This MCT is turned on by applying a positive voltage to the second gate electrode 12, which is connected to the second gate terminal G2, with a voltage that is positive relative to T2 applied to the main terminal T1. Then, an inversion layer is formed on the surface of the (p) base region 9 directly below the second gate electrode 12, and electrons (e) flow into the n- layer 2 via the inversion layer and then into the p+ drain region 22.
That is, the structure including the (p) base region 9 on the cathode electrode side of the MCT controls the injection of majority carriers. This current corresponds to the base current of a pnp transistor having the (p) base region 9 as a collector, the n- layer 2 as a base, and the p+ drain region 22 as an emitter. Therefore, when this bipolar transistor is turned on, a collector current flows from emitter (p+ drain region 22) to collector (p-type (p) base region 9).
The current flows from p+ drain region 22 to (p) base region 9 to conduct electricity between the main terminals T1 and T2. As in the IGBT discussed above in FIG. 8, holes (h) that are minority carriers are injected from p+ drain region 22 to n- layer 2, causing conductivity modulation to reduce the ON-state voltage during conduction. The MCT is turned off by applying a negative voltage to the gate electrode 12.
As a result, the inversion layer on the surface of the (p) base region 9 vanishes, and at the same time, an inversion layer is formed on the surface of the (n) base region 14 directly below the second gate electrode 12. Thus, the (p) base region 9 is shorted with the cathode electrode 13 via the p+ cathode region 15 to stop electrons (e) from flowing from the (n) base region 14 into the n- layer, thereby turning off the MCT.
As described above, although the IGBT can be easily turned on or off using the insulation type second gate electrode with a high input impedance, and allows the ON-voltage to be reduced during the ON state using the conductivity modulation of the n- layer 2, many carriers that have contributed to conductivity modulation thus far must be removed from the n- layer 2 to widen the depletion layer.
Thus, it takes a long time to remove the carriers, resulting in a substantially long turn-off time and thus an increase in switching loss during the OFF state. Particularly, if a current that is an inductive load is cut off, the inductance attempts to maintain the original current and causes a large counter-electromotive force to be applied to the element. As a result, the depletion layer widens and a current draining the electrons (e) starts to flow. This current acts as the base current of a pnp transistor having the (p) base region 9 as a collector, the n- layer 2 as a base, and the p+ drain region 22 as an emitter to flow into the p+ drain region 22.
In conventional devices, a collector current flows during OFF state. That is, holes (h) that are minority carriers continue to be injected from p+ drain region 22 to n- layer 2 to increase the turn-off time. In addition, switching loss during this state increases substantially. Since switching loss occurs each time switching is performed, it has a significant adverse effect on applications involving high-frequency circuits, which requires fast switching of the IGBT, and may nullify one valuable advantage of the IGBT, namely that the steady-state loss is small because the ON-state voltage is small.
In order to improve such turn-off characteristics, the lifetime of carriers can be reduced to accelerate their removal by diffusing heavy metals such as gold and platinum, or by providing irradiation with a radiation source such as an electron beam. These techniques are referred to as `lifetime killers`.
However, the introduction of a life-time killer has the negative effect of increasing the ON-state voltage and has no effect on the continued injection of minority carriers during the OFF state. The problems described above in conjunction with the IGBT also occur in the MCT.
Although the above example is described in conjunction with a lateral semiconductor device suitable for incorporation into an integrated circuit wherein an n-type high-resistivity semiconductor layer is formed on a p-type silicon substrate, the above operation is not limited to this structure. It is also applicable to a lateral or a vertical semiconductor device on an n-type substrate. In addition to the IGBT and MCT, it is also applicable to other semiconductor devices that provide conductivity modulation when minority carriers are injected.